As set forth in U.S. Pat. No. 4,052,648 of Nola, the current through the windings of an induction motor lags behind the applied voltage by an angular amount. The cosine of the angle is called the power factor.
A motor will operate most efficiently at one particular power factor or angular relationship between voltage and current. For the motor in the Nola patent, this optimal amount is 30.degree.. However, as the load on the motor decreases, the lag increases and the motor becomes inefficient. Thus, induction motors operating under light loads waste energy. Given the existence of several million electric motors, as stated in U.S. Pat. No. 4,426,609 of Nagy et al., it can be appreciated that the saving of even a little energy in each motor can represent a significant amount.
Nola, in U.S. Pat. No. 4,052,648, proposed that this energy be saved by controlling the power factor of the energy supplied to the motor so that it remains near its optimal condition, even when the load is reduced. According to Nola, the power factor is controlled by locating an electronic switch, e.g. a triac or SCR, in series between the power line supply voltage and the motor. In addition, various circuits are used to measure the phase difference between the voltage applied to the motor and the current through it. This phase difference is compared to a reference, and a resultant signal is used to control the point in a cycle of applied voltage when the electronic switch is closed. For a sensed increasing phase angle (decreasing power factor) between the motor voltage and current, Nola's circuit shifts the "firing point" or electronic switch closure away from the line voltage zero crossing point so as to apply a smaller portion of each half cycle of the line voltages to the motor. This has the effect of decreasing the phase angle (increasing the power factor) and reducing the power loss in the motor, which is in the form of heat caused by reactive current.
Numerous companies have manufactured power factor controllers according to the Nola patent. However, the efficiency and improvement has yet to be as great as expected, and there have been problems in adapting the circuit to three-phase operation. One of the most notable problems is the unreliability of the circuits, i.e. their tendency to self-destruct Also, it was believed that the three-phase units had problems because the phases interfered with each other.
Circuits exemplary of the improvements made to the basic Nola arrangement are illustrated in Nola's U.S. Pat. No. 4,266,177. This Nola circuit has a "snap-on" feature that applies full voltage to the motor when there are sudden increases in load. This design also has a delay circuit that applies full voltage to the motor during start-up.
U.S. Pat. No. 4,287,464 of Lee describes a "snubber" network which is placed across the electronic switch to reduce spikes that could damage the switch. Harlow's U.S. Pat. No. 4,387,329 describes a three-phase power factor circuit where the control signal from one phase is used to control the cther phases. Optical isolators are used in this circuit to keep the phases from interfering with each other. Similar optical isolators are also disclosed in Nola's U.S. Pat. No. 4,459,528, and isolation transformers for the same purpose are disclosed in Nola's U.S. Pat. Nos. 4,469,998, 4,433,276 of Nola shows a three-phase circuit where the control signal is derived by summing the measured power factor of all three phases.
Another improved circuit is disclosed in a document entitled "Improved Power-Factor Controller", NASA Technical Support Package, NASA Tech Briefs, Summer 1980, Vol. 5, No. 2, MFS-25323. This improvement eliminates the resistor in series with the motor which generates the motor current signal. Instead, the voltage-current phase lag is measured as the difference between the line voltage and the voltage across the electronic switch.
Despite these improvements, both single and three-phase power factor controllers in the prior art do not provide the power savings which might be expected. Further, they are subject to high failure rates.